I'm not sure specifically as there could be many different interpretations in such a fanfiction. It might involve a character named Rito facing a stabbing incident, perhaps in a battle, a crime scene, or a situation of betrayal.
It could be a rogue angel. There are always some angels in the Supernatural universe who oppose Castiel's ideas or actions, and they might see stabbing him as a way to get rid of him or send a message.
In such fanfiction, characters might show shock. For example, Dean could be horrified when he sees Castiel stabbed, his eyes wide open and his face pale.
Well, in 'naruto gets stabbed fanfiction', it could be that Naruto is exploring a new and dangerous area where he encounters a powerful and hostile ninja. In the heat of the fight, this ninja manages to land a stabbing blow. It might also be that Naruto is weakened due to some prior injury or chakra depletion, making him more vulnerable to being stabbed by a regular enemy. And, there could be a situation where Naruto is under the influence of some sort of genjutsu that impairs his ability to defend himself and thus gets stabbed.
Perhaps he gets stabbed as a part of a plot to take over his company. His rivals could be behind it, seeing his success as a threat. In the fanfiction, this could lead to a great revenge story where Kaiba has to regain his power and seek justice.
It might be a jealous rival. If Sakura is popular or has something that others envy, like a special talent or a relationship, a rival could have stabbed her out of envy.
Lala is one of the main characters who interacts with Rito. She is an alien princess and has a very unique relationship with Rito. Their interactions are often filled with her zany ideas and her love for Rito, which sometimes gets him into trouble but also makes for some really fun and romantic moments.
In 'naruto gets stabbed fanfiction', it might be a new enemy that has emerged, someone with a powerful and unknown jutsu. This enemy could see Naruto as a threat to their plans and decide to stab him to eliminate him. Or it could be a character who has been corrupted by dark power, like a former friend or ally who has fallen to the dark side and no longer recognizes Naruto as a friend.
It could be an old enemy of Nicole's from her police work. Maybe someone she put in jail before who has a vendetta against her. This person might have been released recently and is out for revenge.